To cut costs, a high school modified its airconditioning system to

EXPLANATION FROM Fox LSAT

How do we know there wasn’t a rampant gonorrhea outbreak among the students at Skank Unified High that same day? Or how do we know that the AC didn’t happen to get installed right before final exams, when everyone invents bogus illnesses and goes to the nurse? Or how do we know the school bus didn’t get in a 50-car pileup, causing massive injuries among the students and requiring nurse visits?

Yes, the humidity dropped. Yes, nurse visits increased. But we have no proof that one thing caused the other. Correlation does not prove causation.

We’re asked to identify an assumption. I think it will be something that points out that correlation doesn’t necessarily prove causation.

A) Well yeah. If this isn’t true, it becomes, “None of the visits to the school nurse after the AC system was modified were due to illness.” If that’s true, then how can it be true that the AC change caused illness? This must be true in order for the argument to make sense. Therefore it is a Necessary Assumption of the argument.

B) No, it’s not necessary that “most” students suffered in order for the argument to make sense. Even if “less than half” of the students got sick, the illness could still have been caused by the AC. So this isn’t necessary.

C) Who said anything at all about viruses? There is no way this was assumed by the speaker. Nobody is even talking about it. (There are all sorts of illnesses that have nothing to do with viruses.)

D) This strengthens the argument. But it’s so specific that there’s no way it’s a necessary support for the argument. What if a decrease of 18 percent in air humidity causes an increase of 2500 percent in one’s probability of getting ill? If that’s true, then D is false. But the argument would still make sense. On a Necessary Assumption question, the opposite of the correct answer choice should kill the argument. That’s how you know something is necessary! Example: Oxygen is necessary, because without it you die. Food is also necessary. But pizza is not necessary, because without pizza you can still survive.

E) Costs are irrelevant.

Our answer is A.

The option says that the most students suffered from the humidity. If you want to quickly guess whether the option is right or wrong, simply scrutinise each and every word.

Here—>
It said that students suffered, now we dont know whether they suffered enough to go to doctor or what kind of suffering was there? It can be mild discomfort also.
So that’s how we cannot be sure of this answer option.


+kudos if you like my reply. Looking for kudos?

Posted from GMAT ToolKit

Posted from GMAT ToolKit

can anyone please explain why "b" is not the correct answrer?

Most = > 51%

Now, the stimulus states that only 25 % went to the nurse.... What about the others ? Thus this assumption doesnt hold good.

I understand your point. But we are looking here for an assumption. We already know that they have concluded that illness is due to humidity and that is what we have to support now.
Hence, when in A it says that the increase in students was due to illness it clearly connects the dots. In B it just says that people suffered but did they suffer enough to be called ill!!

Hope it helps!

+kudos if my reply helped you.

Posted from GMAT ToolKit

Posted from GMAT ToolKit

In my opinion, the conclusion is people can fell ill due to decrease in Humidity. In A though it says that they came due to illness, but illness due to what is not explained. That makes A fishy, whereas if 25% increase is all because of Illness due to moisture, then it doesn't matter how many or most of the students are suffering.

IMHO thats the missing piece of information/assumption....

Visit to the nurse can be for illness or first aid as well , here the condition ilness along with the information presented in the stimulus propells us to mark (A) as the best available option.

I understand your point. But we are looking here for an assumption. We already know that they have concluded that illness is due to humidity and that is what we have to support now.
Hence, when in A it says that the increase in students was due to illness it clearly connects the dots. In B it just says that people suffered but did they suffer enough to be called ill!!

Hope it helps!

+kudos if my reply helped you.

Posted from GMAT ToolKit

Posted from GMAT ToolKit[/quote]


The intensity of suffering is not d point here ....

A good only just say they suffer ....or have illness.... We don't know whether it's because of humidity or due to some other reason ....

Where in "b" it explicitly stated that it's due to humidity ....


Anyone plz clarify why "a" is dominating over "b"

Posted from my mobile device

I doubt (A) being the correct answer, it only says at least some. Had this been the inference questions this would be correct.

Conclusion: A decrease in humidity can make people ill
Prem: Decrease in humidity of 18% saw a 25% reported increase in #visits to school nurse 24 hrs after decrease

Gap: The gap in logic is the fact that the conclusion drawn states that people were ill - we otherwise have no other information about why people went to the nurse, so the obvious answer is A as it needs to be true for the conclusion to be true.

You can negate this answer choice to show that it is in fact a required assumption.

None of the student vists to the school nurse after the system was modified were due to illness.

Weakens the argument completely, hence it must be required for the conclusion to hold true.

here is why the other answers are wrong:
B is incorrect because it does not need to be true in order for the argument to be true. This is a scope shift. The argument at hand is concerned only with those students who visited the nurse.

C is incorrect because it does not bridge the gap in logic. We need the assumption to draw a link between ill students and the increase in those who visited the nurse. C is a fact that may or may not have any effect on the argument because we don't know whether the students came down with a virus. We just know that they are ill - hence the need to draw a logical link between the illness and the students who visited the nurse.

D is incorrect because it has no bearing on the argument at hand. In fact, D is trying to bait a reader by using the same numbers presented in the stimulus. The fact that the probability increases by 25% does not change the fact that there was a 25% increase in the number of students and does not impact the cause-effect relationship between the humidity change and increase in the number of visits.

E is incorrect because the argument is concerned with establishing a cause-effect relationship between the drop in humidity and the increase in number of visitors. We don't care about costs as it has zero effect on the argument at hand.

Hi Lakshya

Let us analyse the conclusion and the premises on which it is based:

Conclusion: A decrease in humidity can make people ill.

Premise: Twenty-four hours after the decrease in air humidity, a 25 percent increase in the number of visits to the school nurse was reported.

If you notice, there is a subtle language shift between the premise and the conclusion. The premise talks about visits to the nurse and the conclusion talks about making people ill. The underlying assumption, therefore, has to be that the two are connected. Option (A) states exactly this.

Alternatively, you could try to negate option (A).

Negated (A): None of the visits to the school nurse after the system was modified were due to illness.

If none of the visits were due to illness, then the 25% increase was also not due to illness. This negates the conclusion that decrease in humidity makes people ill (based on the premise of increased nurse visits), since none of the visits were due to illness.

Hope this helps.

This is from LSAT. I just took that LR & I actually struggled with this one. People come preaching negation but what is ‘at least some’? Is it 25%?
Also, what if those students were falling sick before and shouldn’t really count in the statistics- 25%. That’s why I chose C. I did a bit of overthinking here but ‘percentage’ & at least some throws me off. There are complex lsat questions where negation doesn’t really work. So, I don’t use it blindly for every ‘assumption’. Honestly, I don’t really have any set in stone technique I use; I just collect information and analyze. This one is simple but weird

Posted from my mobile device

Adding to that, I can probably say that even if C is true (necessary), some people really have to be sick to show up in statistics. So A kinda ‘trumps’ C because it is ‘must be true’.

Posted from my mobile device

(A) At least some of the visits to the school nurse after the system was modified were due to illness.
only if this happens can we confirm or deny the fact that students are becoming ill

(B) Most of the students at the high school suffered from the decrease in air humidity.
May not be the case the nurses came into have a selected complicated case were more assistance was required

(C) It takes 24 hours after a person is infected with a virus for that person to exhibit symptoms.
May not be the case we are not sure about the working of the diseaese or the aboce mentioned timings were caused due to the delay in nurses

(D) A decrease of 18 percent in air humidity causes an increase of 25 percent in one’s probability of becoming ill.
Oversimplification we cannot accurately fit this data

(E) Modifying the air-conditioning system proved to be an ineffective way to cut costs.
This may or may not be the case therefore out

Pr: Decrease Humid. --> Increase # Visit to nurse
Conc: DH --> Increase illness.
==> A big gap need to connect:
#Visit to nurse ---> Illness

Only A is good enough to connect the gap
At least some of the visits to the school nurse after the system was modified were due to illness.

This is too obvious to believe. We all know that a visit to nurse suggests students must have got ill.
B - While this can be a possibility but we can't be sure that it is necessary. If it's true then this is the answer but if not then it is not.
C - It seems to suggest that 24 hour is some sort of criteria or the minimum time for falling ill but it can be possible that students got infected before that, however, they visited after 24 hours.
D - This is an obvious elimination for that it makes too bold a statement with those figures.
E - Irrelevant.

A - This is the right answer but people might think this is already given in the passage while it is not - it is only concluded that they fell ill. This choice broadens the CR ability that there's a fine line between an assumption and a conclusion that is based upon that. It's touches upon the obvious part of our real life i.e. we only visit doctors when we fall ill.

On the other hand, if someone is good at negating the option, he/she might be able to pick it right away.

Answer A.

Hello from the GMAT Club VerbalBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.